A hydrogen absorption alloy is an alloy that can stably and easily storage hydrogen as an energy source, and is receiving enormous attention as a new material for conversion and storage of energy.
Regarding application fields of the hydrogen absorption alloy as a functional material, there have been proposed a wide range of applications such as storage and/or transportation of hydrogen, storage and/or transportation of heat, conversion of energy between heat and a machine, separation and/or purification of hydrogen, separation of hydrogen isotopes, batteries with hydrogen as an active material, catalysts in synthetic chemical and temperature sensors.
For example, a nickel-metal hydride rechargeable battery using a hydrogen absorption alloy as a negative electrode material has advantages of (a) high capacity, (b) high resistance to overcharge and overdischarge, (c) being capable of high rate charge-discharge, (d) being clean and the like, and therefore receives attention as a consumer battery, and activities for its practicalization and application are vigorously conducted.
Thus, the hydrogen absorption alloy has potential for a variety of applications from mechanical, physical and chemical viewpoints, and is listed as one of key materials in future industries.
As electrode materials for a nickel-metal hydride rechargeable battery, which represents one application example of such a hydrogen absorption alloy, AB5 type rare earth-Ni-based alloys having a CaCu5 type crystal structure have been so far put into practical use, but these alloys reach the limit at a discharge capacity of about 300 mAh/g, and it is difficult to further increase the capacity.
In contrast, rare earth-Mg—Ni-based alloys, the capacity of which can be increased, have been receiving attention in recent years. These alloys have mutually different complicated layered structures, and are reported to show a discharge capacity greater than that of the AB5 type alloy when used for electrodes, and expected as a next-generation negative electrode material for a nickel-metal hydride battery.
For improving cycle performance while retaining a high discharge capacity of such a rare earth-Mg—Ni-based alloy, the kind and amount of a metal that is further added to the hydrogen absorption alloy are adjusted (Patent Documents 1 to 4).